The structure external-calls has
most of the Scheme functions described here.
The others are in
dynamic-externals, which has the functions for dynamic loading and
name lookup from
the section on Dynamic Loading,
and shared-bindings, which has the additional shared-binding functions
described in
the section on the complete shared-binding interface.

The names of all of Scheme 48's visible C bindings begin
with `s48_' (for procedures and variables) or
`S48_' (for macros).
Whenever a C name is derived from a Scheme identifier, we
replace `-' with `_' and convert letters to lowercase
for procedures and uppercase for macros.
A final `?' converted to `_p' (`_P' in C macro names).
A final `!' is dropped.
Thus the C macro for Scheme's pair? is S48_PAIR_P and
the one for set-car! is S48_SET_CAR.
Procedures and macros that do not check the types of their arguments
have `unsafe' in their names.

All of the C functions and macros described have prototypes or definitions
in the file c/scheme48.h.
The C type for Scheme values is defined there to be s48_value.

Scheme 48 uses a copying garbage collector.
The collector must be able to locate all references
to objects allocated in the Scheme 48 heap in order to ensure that
storage is not reclaimed prematurely and to update references to objects
moved by the collector.
The garbage collector may run whenever an object is allocated in the heap.
C variables whose values are Scheme 48 objects and which are live across
heap allocation calls need to be registered with
the
garbage collector.